1. Field of the Invention
The present invention relates to a honeycomb structure for use in a high temperature environment and to a method of manufacturing the same, and more particularly a honeycomb structure which carries a catalyst as a support matrix for use in a catalytic converter for purifying the exhaust gas from an automobile engine and to a method of manufacturing the same.
2. Prior Art
The honeycomb structure of the conventional type which has been used is usually composed of a strip of corrugated plate which is made by folding the plate into continuously corrugated unevenness and of a strip of flat plate, which are alternately disposed and welded to each other by brazing into multilayers, and made overall into a prescribed configuration in a form such as a wound multilayered block. As the base metal of the above, a corrugated plate and a flat plate each having a uniform thickness have generally been used. In order to withstand use in a high temperature environment, in many cases stainless steel foil or the like have been used, and for the same purpose a brazing filler metal from, for example, the nickel group and the iron group for the brazing filler metal has been used. The honeycomb structure of this type has been used in high temperature environments, particularly in environments subject to thermal cycles of repeated heating and cooling. For example, this honeycomb structure has been used in catalytic converters for purifying the exhaust gas from automobile engines, for carrying the catalyst on its corrugated and flat plates which serve as the support matrix, and the high temperature exhaust gases from the running automobile engine including noxious emissions have been purified through reaction with the catalyst which is effected by passing through the honeycomb structure.
As examples of the conventional honeycomb structure of this type, there are U.S. Pat. No. 5,084,361, 4,282,186 and 4,400,860, and B.P. 1,452,982, applied for by the present inventor.
However, there is the following problem which has been realized about the conventional honeycomb structure of this type. When this honeycomb structure is used as the catalyst support for an exhaust gas purifying apparatus, the temperature of the exhaust gas from the running engine passing through the honeycomb structure is uniformly elevated, and the whole body of the honeycomb structure, i.e., both its outer side or casing side and its inner side or center portion, is heated by the high temperature exhaust gas. Since the outer side of the honeycomb structure is cooled by open air through the casing, its temperature rise when heated is relatively small; but the honeycomb structure is rapidly cooled to a low temperature when the engine of the vehicle is stopped, while the inner side of the honeycomb structure is not cooled by open air, and the temperature rises to a high level when heated and is maintained at that high level for a long time even when the engine of the vehicle is stopped. In the honeycomb structure which is used in a high temperature environment like this, particularly in an environment subject to thermal cycles of repeated heating and cooling, a difference between the amounts of thermal expansion and contraction takes place, as a result of which stress is generated between the outer and inner sides of the honeycomb structure due to the large temperature difference existing therein.
FIG. 1a is an enlarged view showing the principal part of the outer side of the honeycomb structure of the conventional type. In the figure, flat plate 1 is welded to corrugated plate 3 by means of brazing filler metal 2 applied to both sides thereof. Corrugated plate 3 can absorb stress A by deformation; however, since flat plate 1 is fixed at the weld both sides, it is hard to absorb stress A, particularly in the range of the first and second layers from the outside thereof, and cracks B and exfoliation are liable to be generated in the area of the weld due to stress A concentrated thereto, and problems have been observed from the viewpoint of heat proofness, durability and reliability of the honeycomb structure. Hitherto, countermeasures have been taken to prevent concentration of stress A by limiting the location of welding flat plate 1 and corrugated plate 3 to only a part of the total contact line or by providing thicker flat plate 1; however, no fundamental solution has been obtained, and stress A is still concentrated in the area of the weld of flat plate 1, and cracks B and exfoliation are also still liable to be generated.
FIG. 1b is a perspective diagram showing the direction of stress A in the area of the outer side of the conventional type of honeycomb structure. When the honeycomb structure is used as the catalyst support for an exhaust gas purifying apparatus, a large temperature difference is generated as described above between the outer side of the honeycomb structure on the side of casing 4 and the inner side thereof on the side of the center; further, since casing 4 is thicker and of a different material than flat plate 1 and corrugated plate 3, thermal expansion and contraction due to heating and cooling thereof commonly take place but with different amounts on one side, i.e., in casing 4, the outer side of the honeycomb structure, and on the other side, i.e., on the inner side thereof, resulting in stress A working in the reverse direction on each side. For example, when the honeycomb structure is heated, the direction of thermal expansion, i.e., elongation, which occurs is the same as the passing direction of exhaust gas C, however, the amount is small with the outer side former but large with the inside. As shown in FIG. 1b, demarcating the range at the first or second layer from the casing 4 and at the inner side of the honeycomb structure, i.e., in the former, stress A works in the reverse direction of passing exhaust gas C, while in the latter, stress A works in the direction of exhaust gas C. Therefore, on the outer side of the honeycomb structure, in particular in the range of the first and second layers from the outside, stress A concentrates in the area of the weld of flat plate 1 and corrugated plate 3, whereby the core is sometimes caused to slip out, and problems of the honey-comb structure are observed from the view-point of heat proofness, durability and reliability.